Hi,

You'll find some reasons on the w3c website

https://validator.w3.org/docs/why.html

thank you Pierre, but these are all the mentioned "well known(common)" reasons :)  my point was that the web is machine oriented and having "clean" markup will ease the job of  "crawlers"  for instance  :) any other thoughts?

If this : http://www.buckleupstudios.com/blog/w3c-web-standards-benefits/ can be used as "well known (common) reasons" I would add only this:  Reducing JavaScript errors (if used to manipulate DOM) 

I believe that It is how to ensure that your system will remain compatible with user agents over time

Igor, Rodrigo Thank you!

Igor: could you please be more specified on JS error reducing option?

Rodrigo: sounds optimistic :)

Web standards are made for set of rules and technical specifications for website code...So follow the w3c standard gives common base...a foundation for the world wide web so that browsers and other software understand the same basic standard. 

Suresh, Shweta, Thank you 

The problem started when companies like Microsoft introduced their own standards, which only MS Internet Explorer was able to understand. I still remember that I tried to write a simple HTML code with JavaScript code that worked fine with Netscape while Internet Explorer could not render it correctly. By following the standards one can be sure his web content will be rendered correctly no matter what browser or operating system one uses.

Thanks Apostolos, I do remember that time too... I had to write 2 versions of web page one for Netscape and one for IE, the so called  "browser war"  time :) W3C standards if you remember were introduced to be kind of obligatory for everyone, but there were (still is but changing) no mechanism forcing people do what was recommended to be done... so all this "promotions" to keep standards and promises that you'll rank better (SEO stuff) is kind of "instruments" to make people less mess up their code... and as a result better structure  and index the content of web...